Electrical amplifier circuit



Feb. 17, 1931. L M HULL l 1,792,984

ELECTRICAL 'AMPLIFIER C I-RCUIT Feb. 17, 1931-. L. M HULL .1,792,984

ELECTR I CAL AMPLIFIER C IRCUIT Filed July 14. 1927 2 sheets-sheet 2 Patented Feb. 17, 1931V UNITED STATES PATENTJOFFICE i 'i LEWIS M. HULL, OF BOONTON, NEVI JERSEY, 'ASSIGNGR .IO RALO FREQUENCY LABORA- 'ronrn's INCORPORATED,

or :BooNrcN y JERSEY Naw cansar, a 'coarona'rroN or NEW vnrincrnrcar. alarma-Ina cincnrr Application inea July A11i, 19a?.V serial No. 205,733.

This invention relates to electrical amplifier circuits of the type employing vacuum tubes of thetype having an electron-emitting cathode, an anode, and one or more additional elements orgrids.` Itprovides a balanced amplier circuit for such vacuum tubes, whereby retroaction caused by coupling due to capacities between the vacuum tube elements and their Vassociated connect-ions, and especially retroaction caused by coupling due to the capacity between the control-grid andv the anode, is reduced or eliminated.

Advantageous features of my invention are that the tuning condensers may be directly connected to ground (i. e. to theframework, or low-potential portion of the system, which is normallygrounded),Y thus producing structural simplicityof the amplifier, in practical embodiments thereof; that the A and B batteries may still be grounded asshown; that the compensating condenser, if used, may be connected directly between the control grid and ground or low-potential part of the lsystem; and that simultaneously with the above arrangements the` input to the stage may be connected directly between the cathode and the control grid of the vacuum tube. Y

Figure l shows my invention applied to a single amplifier stage. Figs. 2, 3 anda show variations of the arrangement of Fig. l.

Figs. 5 and 6 show cascaded amplifiers withl arrangements similar to Figs. spectively.

In Fig. 1,as shown, I employ a threeelectrode vacuum tube of the. ordina-ry kind, having anode l?, cathode F, and control grid, Gr. The input to the amplifier stage,

1 and 2, re-

from whatever source or variety of circuit V- may be desired, is suitablyconnected directly between Gr and F. Thecathode, or lilament, F, may be heated from the direct current source A. In either, or preferably in each, of the connections from the cathode to this source, is included a coil. Coils in each connection are-here shown at L3 and L4. The coil, or coils, are coupled to the plate coil L2. If two coils are used, they shouldbe so arranged as to have substantially equal mutual inductance with the coil LaV They are preferably equal to each other as well, andthe inductance of each should ordinarily be subi stantially less than 'that of the plate coil L2.

This may be accomplished by means of a twinwinding, the parallel strands being laid or bonded together. For certain purposes it is desirable that the` coils L3 and L, `(i. e.

the twin winding) shall be coupledV to the coil L2 with a coeicient of coupling substantially equal to unity. In this case it is obvious that the inductance of coils L3 and L4 must be substantially less than that of the coil L2, so that they will not prevent the plate current of the vacuum tube `fromftransferring energy into the tuned circuit L2-C2. With a particular standard type of audion tube I have found that good results are obtained when the coil L2 has 60 turns and the twin Vwinding, which constitutesthe coils L3 and Li, "has -20 turns. These figures, however, are merely `given by `way of example, 'and the invention is not limited-thereby.- j

The coils L2', Laand L4 should all be wound Y in the same sense. The `coil L2 maybe tuned by means of the variable condenser C2. The

lower' end of the coil L2 is grounded through the batteries A and B, which may be bypassed by the condenser C1.v A compensating condenser, C1, which improves the bal- Vancing faction of the circuit, may be connected between the grid and ground, if desired, and` Ii often find. it advisable to do so.

- A variation of this arrangement is shown in Figure 2, in which the tuning condenser C2 isconnected directly to ground, which is ordinarily the frame of the set as described above.` The resonant circuit`L2-G2 may be closed through the by-pass condenser C1.

Another arrangement is shown in Fig. 3, in which the coil L., has-the form of a radio frequency choke coil. It `should have a high self-inductance in comparison withthe coil L3', and is not coupled to either L2 `or L3. L3,

however, is coupled to L2, as before, and may have substantially unity coupling therewith. An arrangement including a double-grid vacuum tube is shown in Fig. 4. The circuit is similar to that of Fig. 2, but includes,lin addition, the second grid, G2, which may be ccnnectedY to ground as regards alternating current, preferably by means 0f the by-pass Till condenser C11. The grid G2 may be given a positive direct current bias by means of a connection to a desired point in the B battery, or otherwise. Under certain circumstances, for instance if the tube elements have the proper geometrical arrangement, or the coils are properly proportioned, the circuit may be self-compensating, probably because of the natural capacity between grids G1 and G2. This is shown by the dotted lines at C1.'

However, an external capacity may be used at Cl if, for any reason, it is not desired to make use of the natural capacity alone.

Figure 5 shows a cascaded arrangement employing a circuit similar to that of Fig. l, but in which the coilLG serves as the plate coil, for the purpose of coupling the tuned circuit Lg---C2 to the anode-cathodecircuit, instead of using the coil L2 itself as a plate coil, as in the previous arrangements. An at vantage yof this arrangement is that the coupling ratio may be varied in order to fit the impedance of the tuned circuit lag-C2 to the impedance of the vacuum tube, whether the tube be of a single-grid or double-grid type. By making the impedance of the tuned circuit of substantially the same order as the tube impedance, greater efficiency and better operation is possible.

Figure 6 shows another cascaded arrangement, employing a circuit similar to that of Fig. 2, except that double-grid vacuum tubes are employed. Coupling between the stages is effected by means of coils L5 coupled to the tuned plate coils L2. Y l

From the foregoing specific examples, it will be seen that it is characteristic of my in vention that I connect the input circuit of the amplifier stage directly between the control grid and the cathode of the vacuum tube, and that I may provide balancing capacity, if used, so as to be effectively between the control grid and ground, or between the control grid and a second grid in the case of a double-grid vacuum tube, said second grid being so related to ground as to cause capacity effectively between the control grid and ground. When either of these arrangements is employed, whether the vacuum tube is provided with one or more grids, it is covered by the expression capacity effective between control grid and ground or the like, as herein employed.

Any or all of these electrical amplifier stages are of course suitably used in radio receiving sets incombination with other radio or intermediate frequency amplifier stages, detectors, or audio frequency amplifiers. Certain combinations or variations of the circuits here shown, and modifications thereof, fall, of course, within my invention.

All degrees of coupling, whether unity coupling or substantially less than unity coupling, are comprehended by the term coupled or the like, as herein employed.

I claim:

l. An electrical amplifier stage comprising, in combination, a vacuum tube including an anode, a cathode, a control grid, and a second grid, input terminals for the amplifier stage constituted by said cathode and said control grid, a ground for said amplifier stage, a plate coil included in the circuit beween said anode and said ground, a coil included in a connection between said cathode and said ground and coupled to said plate coil, and capacity effective betweensaid two grids, whereby the elements of said amplifier stage may cooperate to reduce retroaction.

2. An electrical amplifier comprising, in combination, a vacuum tube including at least an anode, a cathode, and a control element; an input circuit for said amplifier connected between said cathode and said control element of sai-d tube; an output circuit for said amplifier including a coil connected between said anode of said tube and ground; a coil connected between said cathode of said tube and ground and coupled to said coil in said output circuit; and balancing capacity connected and effective between said control element of said tube and ground; said ampli.- fier circuit being so proportioned and arranged as to be susceptible of balance to reduce retroaction between said input and output circuits thereof.

In testimony whereof, I affix my signature.

LEWIS M. HULL. 

